The present invention relates to ethylene copolymers and in particular to ethylene copolymers provided with a uniform distribution of the comonomer units within the polymer chain and with a wide molecular weight distribution. This invention also relates to a process for the preparation of ethylene-based polymers and to the catalytic system used in this process.
It is known that polyethylene can be modified by the addition, during the polymerization reaction, of small quantities of xcex1-olefins, generally 1-butene, 1-hexene or 1-octene. This gives LLDPE (linear low density polyethylene) copolymers which have short branches along the main chain due to the units derived from the xcex1-olefin comonomers.
These branches have the effect that the degree of crystallinity and hence the density of the copolymer turn out to be lower than in the polyethylene homopolymer. Typically, the LLDPE copolymers have densities of the order of 0.910-0.940 g/cm3. Consequently, the LLDPE copolymers have optimum mechanical properties, in particular for the production of films.
The lowering of the degree of crystallinity and of the density of the copolymers is a function of the type and quantity of the incorporated xcex1-olefin. In particular, the greater the quantity of incorporated xcex1-olefin, the lower are the resulting degrees of crystallinity and density.
The properties of the copolymer depend, apart from the type and quantity of the incorporated xcex1-olefin comonomer, also oil the distribution of the branches along the polymer chain. In particular, the greater uniformity of the distribution of the branches has a positive effect on the properties of the copolymers. In fact, with the same type and quantity of incorporated xcex1-olefin, a higher uniformity of distribution allows lower degrees of crystallinity and density to be obtained.
LLDPE copolymers prepared with conventional Ziegler-Natta catalysts are characterized by a poor uniformity of composition distribution, and in particular by the presence of consecutive units of comonomers in the polymer chain and by long sequences of ethylene units. To obtain copolymers with sufficiently low density and crystallinity, it is therefore necessary to use large quantities of xcex1-olefin comonomer.
The use of catalysts based on metallocene compounds has made it possible to obtain LLDPE copolymers having an improved uniformity of composition. These copolymers, of the same chemical composition, are endowed with superior properties as compared with traditional copolymers.
The copolymers obtained with metallocene catalysts have, however, a narrow molecular weight distribution (MWD). This expresses itself in poor processability properties, and this can be disadvantageous in particular in some applications, such as the use for films.
With the purpose of overcoming this disadvantage, processes for the preparation of LLDPE copolymers have been proposed, which are carried out in the presence of a catalyst consisting of a metallocene compound and a non-metallocene titanium compound.
U.S. Pat. No. 4,701,432 describes, for example, the preparation of LLDPE in the presence of a catalytic component consisting of bis(cyclopentadienyl)zirconium dichloride and titanium tetrachloride, supported on silica.
European Patent Application EP 439,964 describes the preparation of LLDPE in the presence of a catalyst consisting of a metallocene compound and a compound based on titanium, magnesium and a halogen.
Nevertheless, the LLDPE copolymers thus obtained, though having a broad molecular weight distribution, do not have a satisfactory distribution of the comonomers within the polymer chain.
Another proposed system is that which provides for the use of catalysts consisting of two different metallocene compounds.
K. Heiland and W. Kaminsky in xe2x80x9cMakromol. Chem. 193, 601-610 (1992)xe2x80x9d describe the copolymerization reaction of ethylene with 1-butene in the presence of a catalyst consisting of a mixture of rac-ethylenebis(indenyl)zirconium dichloride and rac-ethylenebis(indenyl)hafnium dichloride. The copolymers obtained with this catalyst are provided with a substantially bimodal molecular weight distribution. Nevertheless, the regularity in the distribution of the comonomers across the various polymer chains does not yet reach a high level. This is due to the fact that the values of the copolymerization parameters r1 and r2, characteristic of the zirconocene and hafnocene used, differ widely from one another, as reported on page 607 of the above publication.
It would therefore be desirable to obtain an LLDPE copolymer which has an extremely uniform distribution of the comonomer units in the polymer chain and at the same time a broad molecular weight distribution.
Stereorigid metallocenes are generally prepared as a mixture of the racemic and meso form. The meso form is generally removed by separation from the rac/meso mixtures, since only the chiral racemic form is stereospecific. Only a few disclosures are known of the use of the meso form in the polymerization of olefins.
European Patent Application EP 584,609 discloses the use of a particular class of bridged bis-indenyl metallocenes in the meso form or in a meso/rac mixture for the preparation of atactic polyolefins having high molecular weight and a narrow molecular weight distribution (Mw/Mnxe2x89xa64). In particular, homo- and copolymers of propylene were prepared.
European Patent Application EP 643,078 describes the preparation of ethylene-based polymers endowed with very high molecular weights and narrow molecular weight distribution, by carrying out the polymerization reaction in the presence of a metallocene-based catalyst wherein the metallocene is stereorigid and it is in its meso isomeric form.
Surprisingly, the applicant has now found that it is possible to prepare LLDPE copolymers having a uniform distribution of the comonomer units along the polymer chain and a broad molecular weight distribution, as well as other ethylene-based polymers, by carrying out the polymerization reaction in the presence of a catalyst consisting of a mixture of the racemic and meso isomers of a stereorigid metallocene compound.
An object of the present invention is therefore an ethylene copolymer with at least one comonomer selected from:
(a) xcex1-olefins of the formula CH2xe2x95x90CHxe2x80x94CH2R, where R is hydrogen or a linear, branched or cyclic alkyl radical having 1 to 20 carbon atoms,
(b) cycloolefins and
(c) polyenes,
with a content of units derived from xcex1-olefin, cycloolefin and/or polyene comonomers of between 1 and 20 mol %, preferably between 2 and 15 mol %, characterized in that:
(a) in TREF (Temperature Rising Elution Fractionation) analysis, a quantity equal to at least 90% by weight of the copolymer is eluted in a temperature interval of less than 50xc2x0 C., and
(b) Mw/Mn greater than 3, where Mw is the weight-average molecular weight and Mn is the number-average molecular weight, both determined by GPC.